1. Field of Invention
The present invention relates to the use of radiation response measurements for determining the quality and quantity of bulk material in an on-line process. More particularly, this invention pertains to Multi-energy Gamma Attenuation (MGA).
2. Description of the Related Art
Several technologies exist to determine the composition of bulk materials. One that is especially important to the present invention is Dual-energy Gamma Attenuation (DGA) analysis.
Dual-energy Gamma Attenuation (DGA) based sensors have been used for many years. A DGA device operates on the premise that analyzed material will attenuate different energy gamma rays in fixed repeatable ways. A DGA device consists of a gamma energy source arrangement consisting of dual energy gamma emitters. The gamma emitters are chosen in such a way that the material to be analyzed will attenuate the different energy gamma rays in ways that are conducive to measuring one or more specific properties of the material being measured. The DGA technique relies on the fact that material is composed of constituents with a range of atomic numbers and that a distinction between the constituents is advantageous in processing. The attenuation of higher-energy gamma energy is relatively insensitive to the material's atomic number, but the attenuation of lower-energy gamma energy is very sensitive to the material's atomic number. These different attenuation characteristics can strongly differentiate between high and low atomic number material constituents. Two sources are used in DGA analysis: a low-energy source (typically at 60 KeV) and a high-energy source (typically at 662 KeV). The energy attenuation from the two sources resulting from transmission through the analyzed material can then be combined to distinguish between materials of different composition. This technique does not allow for identification of specific elements, but instead only characteristics of the composition. One such application of DGA technology uses gamma ray sources to interrogate coal, with the assumption that the material that coal is composed of will attenuate the differing energy gamma rays to produce a measurement that is conducive to determining coal ash content and density. For other mineral compositions, this technique is used to distinguish between the desired mineral and the undesirable overburden or interburden.
The DGA analysis technique involves bombarding a bulk material with gamma rays from two gamma ray emitters of sufficiently different energies. The gamma rays interact with the bulk material resulting in the attenuation of the number of gamma rays transmitted through the bulk material. The gamma rays are typically detected by a scintillation crystal (typically Nal) monitoring the two energy levels of the sources. The sum of these released gamma rays at these specific energies is referred to as an energy spectrum. The technology relies on the fact that elements with different atomic numbers attenuate gamma rays at specific energies in different ways. Thus, for low-energy gamma rays (i.e., those generated by a low energy gamma emitter such as Am-241), the attenuation of gamma rays is largely dependent on the atomic number of the atoms/elements present in the bulk material. For high-energy gamma rays (i.e., those generated by a high-energy gamma emitter such as Cs-137), attenuation is largely independent of the atoms/elements in the bulk material. Analysis of the energy spectrum leads to a determination of the bulk elemental composition of the bulk material.